1. Field of the Invention
The invention relates to method for frame quality detection used in a radio system comprising a plurality of base stations and subscriber terminals transmitting and receiving signals via different channels, the quality of the signals being measured by the bit error rate, the terminals generating speech frames from voice and silence descriptor frames regularly from silence, the frames comprising bits, and each frame being interleaved with another frame, and each frame being transmitted in a known number of essentially successive bursts comprising a frame start and end identifier and bits that are decoded by soft bit decisions.
The invention also relates to a receiver arranged to be used in a radio system comprising a plurality of base stations and subscriber terminals arranged to transmit and receive signals via different channels, and arranged to measure the quality of the signals by the bit error rate, and comprising at least speech frames and silence descriptor frames, the frames comprising bits and each frame being interleaved with another frame and each frame being arranged to be transmitted in a known number of essentially successive bursts comprising a frame start and stop identifier and bits that are arranged to be decoded by soft bit decisions.
2. Description of Related Art
In a cellular radio system, and especially in the GSM system, discontinuous transmission (DTX) is used to reduce interference and subscriber terminal power consumption. This operates so that when the voice coder of the subscriber terminal observes silence in speech, the subscriber terminal transmits only periodically (once every 480 ms) a silence descriptor frame (SID), typically used for noise generation in a receiving subscriber terminal. Without noise the receiving user finds the silence unpleasant and may even think that the connection has been disconnected.
However, the silence descriptor frame causes problems. In a cellular radio system the channel decoder of a receiver attempts to correct mistakes caused mainly by the air interface between subscribers. However, correction does not always succeed. This means that the channel decoder detects a bad frame (BFI), and the substitution and muting algorithm in the speech decoder "plays" again the last good frame received. If several bad frames succeed each other, the speech decoder "plays" the last good frame received again and again and gradually attenuates it completely. Known bad frame detection methods include e.g. 3 bit CRC (Cyclic Redundancy Check) and the pseudober method. In the CRC method a check parity is calculated for a bit block according to a known code polynome. A bit block usually comprises the most significant bits of a speech block, giving a three-bit parity check. This is compared with the parity bits calculated by the channel coder, and if the parity bits are the same, the frame received is good. In the pseudober method, a received frame is channel coded again with a convolution code after Viterbi decoding. The coded frame is directly compared with a frame received from the channel, and the differences in the bit sequences of the frames indicate error corrections made by the Viterbi decoder. All errors cannot, however, be corrected by the Viterbi decoder. In this method a frame corresponding to e.g. a speech block is determined to be bad if the Viterbi decoder has changed more bits than allowed by a predetermined limit. Another known method of estimating the bit error rate is to calculate the bit error rate from a predetermined training sequence of each traffic channel during a measuring period. If the received signal comprises more erroneous bits than allowed by the predetermined limit, the frame is determined to be bad.
Reception of discontinuous transmission is the most susceptible to errors in detection of bad frames. If a speech decoder determines a bad frame to be good during silence, the speech decoder switches to speech mode and "plays" this false frame. As the frame was erroneously detected, the following frames are likely to be real false frames and the speech decoder starts frame substitution and muting processes with a false frame, generating very unpleasant sounds in the ear of the receiving user.
CHARACTERISTICS OF THE INVENTION
It is an object of the present invention to essentially improve bad frame detection as compared with prior art. Simultaneously, the user friendliness of a subscriber terminal is also improved.
This is achieved with a method described in the preamble, which is characterized in that two results, result A and result B, are compared with each other and with a predetermined threshold value for frame quality detection, result A being formed as the sum of soft bit decisions on one or more bursts comprising bits from successive frames N and N+1, and result B being formed as the sum of soft bit decisions on one or more bursts comprising bits from successive frames N-1 and N, and that when comparing results A and B with each other, frame N is determined to be bad if either result is essentially smaller than the other result, or when comparing results A and B with the predetermined threshold value, frame N is determined to be bad if both results are smaller than the predetermined threshold value.
Another method of the invention is characterized in that two results, result A and result B, are compared with each other and a predetermined threshold value for frame quality detection, result A being formed as the product of soft bit decisions on one or more bursts comprising bits from successive frames N and N+1, and result B being formed as the product of soft bit decisions on one or more bursts comprising bits from successive frames N-1 and N, and that when comparing results A and B with each other, frame N is determined to be bad if either result is essentially smaller than the other, or when comparing results A and B with the predetermined threshold value, frame N is determined to be bad if both results are smaller than the predetermined threshold value.
A third method of the invention is characterized in that two results, result A and result B, are compared with each other and a predetermined threshold value for frame quality detection, result A being formed by calculating the bit error rate from one or more bursts comprising bits from successive frames N and N+1, and result B being formed by calculating the bit error rate from one or more bursts comprising bits from successive frames N-1 and N, and that when comparing results A and B with each other, frame N is determined to be bad if either result is essentially smaller than the other, or when comparing results A and B with the predetermined threshold value, frame N is determined to be bad if both results are greater than the predetermined threshold value.
The invention further relates to a receiver comprising a result unit for creating two results, result A and result B, of which the result unit is arranged to form result A as the sum of soft bit solutions from one or more bursts comprising bits from successive frames N and N+1, and result B as the sum of soft bit decisions on one or more bursts comprising bits from successive frames N-1 and N, and a comparison unit for comparing the two results with each other and with a predetermined threshold value, the comparison unit being arranged to determine frame N to be bad when comparing the results with each other if either result of the comparison unit is essentially smaller than the other result, or when comparing the results with the predetermined threshold value, the comparison unit is arranged to determine frame N to be bad if both results are smaller than the predetermined threshold value.
The invention also relates to a receiver comprising a result unit for creating two results, result A and result B, of which the result unit is arranged to form result A as the product of soft bit solutions from one or more bursts comprising bits from successive frames N and N+1, and result B as the product of soft bit decisions on one or more bursts comprising bits from successive frames N-1 and N, and a comparison unit for comparing the two results with each other and with a predetermined threshold value, the comparison unit being arranged to determine frame N to be bad when comparing the results with each other if either result of the comparison unit is essentially smaller than the other result, or when comparing the results with the predetermined threshold value, the comparison unit is arranged to determine frame N to be bad if both results are smaller than the predetermined threshold value.
The invention also relates to a receiver comprising a result unit for creating two results, result A and result B, of which the result unit is arranged to form result A as the bit error rate of one or more bursts, wherein the bursts comprise bits from successive frames N and N+1, and result B as the bit error rate of one or more bursts, wherein the bursts comprise bits from successive frames N-1 and N, and a comparison unit for comparing the two results with each other and with a predetermined threshold value, the comparison unit being arranged to determine frame N to be bad when comparing the results with each other if either result of the comparison unit is essentially smaller than the other result, or when comparing the results with the predetermined threshold value, the comparison unit is arranged to determine frame N to be bad if both results are greater than the predetermined threshold value.
The method and receiver of the invention provide considerable advantages. The method of the invention essentially reduces erroneous bad frame detection in discontinuous transmission especially when erroneous detection causes a receiver unpleasant sounds. In the method of the invention a relative threshold is used, the receiver automatically being adjusted to varying circumstances in the channel.
The preferable embodiments of the methods of the invention are also disclosed in the attached dependent claims and the preferable embodiments of the receivers of the invention are disclosed in the attached dependent claims related to the receivers.